Radioisotopic heat source

ABSTRACT

A radioisotopic heat source and method for a long life electrical generator. The source includes a mixture of plutonium dioxide shards and yttrium or hafnium enclosed within a container of a particular tantalum-tungsten-hafnium alloy, all being enclosed in a nickel alloy outer container, and subjected to a heat treatment of from about 1570K to about 1720K for about 1 hour.

Jones et al.

[ 1 Sept. 30, 1975 RADIOISOTOPIC HEAT SOURCE 3,600,585 8/1971 Kelly et. a1 250/493 3,697,329 10/1972 Bunker et al. 136/202 [751 Inventors: :32: g t i z l g j g 3,767,930 10/1973 Suyell 250/493 Paul E. Teaney, Dayton, both of Ohio Primary E.\'an1inerArChie R. Borchelt [73] Assignecz The United States of America as Attorney, Agent, or Firm-Dean E. Carlson; Dudley represented by the United States King; Richard Constant Energy Research and Development Administration, Washington, DC. 1221 Filed: May 30, 1974 [57] ABSTRACT [21 Appl. No.: 474,548 A radioisotopic heat source and method for a long life electrical generator. The source includes a mixture of plutonium dioxide shards and yttrium or hafnium en- 250/4935369/83 Closed within a Container of a particular I i 499 S00 tantalum-tungsten-hafnium alloy, all being enclosed in a nickel alloy outer container, and subjected to a heat treatment of from about 1570K to about 1720K for [56] References Cited about 1 hour' UNITED STATES PATENTS 10 Claims, 4 Drawing Figures 3,533,913 10 1970 Botts et al 250/493 x Q 3 12 q 38 Q RADIOISOTOPIC HEAT SOURCE BACKGROUND OF INVENTION Radioisotopic heat sources are used in many applications which require a relatively long life. These uses include providing thermal energy for thermoelectric energy conversion generators, heaters, and the like in space, military, medical and similar applications.

Because of the radiological and toxic nature of many of the radioisotopes used in such heat sources, stringent safety requirements are needed to insure that the radioisotopes will be contained in the heat source under all normal operating conditions as well as any abnormal conditions which may be encountered. When the thermal energy requirements and/or the useful lifetime requirements are increased, the containment of the radioisotopes becomes a greater problem. This containment is also made more difficult when radioisotopes such as plutonium are utilized which adversely react with many container materials, particularly at the operating and other design temperature and environmental conditions to which the heat source may be subjected.

Plutonium radioisotopes, and particularly plutonium- 238, are attractive as long life radioisotope heat sources due to the long half-life of the isotope and its being predominantly an alpha emitter and only a limited emitter of more penetrating radiations. Plutonium, however, is extremely toxic to humans when inhaled at even low quantities and any heat source utilizing same must be designed to withstand the internal temperatures or pressures produced by the raidioisotope itself without adverse reaction therewith, as well as any potential impact or other loads and temperatures which may be provided externally of the heat source.

Many of the conventional high temperature and/or high strength metals or other materials which are normally utilized for containers or impactresistant members may have their containment characteristics adversely affected when utilized in a radioisotope heat source application which may make them unsuitable for such applications. In addition, many of these mate rials are not readily welded or otherwise formed into a desired heat source configuration or are made of or in clude materials which are highly reactive with the radioisotope or other materials commonly utilized therewith.

Radioisotope powered thermoelectric generators are presently being utilized in various space and the like applications. A typical generator, such as that described in U.S. Pat. No. 3,388,008 to R. J. Campana et al. for Thermoelectric Generator and issued June 1 l, 1968, may produce about one milliwatt of electrical power output using wire-type thermocouples. These generators may be limited in their electrical energy output, in the thermal energy which is produced by their heat source, and/or in their lifetimes. \Nhen attempts are made to increase electrical or thermal energies, generator lifetimes are generally decreased due to the more difficult environmental conditions to which the materials which contain the radioisotope are subjected. The increase of lifetime, thermal power and electrical outputs generally suggest the use of highly exotic and expensive materials and processing techniques to manufacture the device which may excessively increase the cost of the heat source possibly making alternate power supplies more attractive. Chemical power sources may be considered, but are generally of much shorter lifetimes than may be desired and require replacement or may not be useable in applications where the power source is not readily accessible. In addition, the chemical power sources are generally relatively large and heavy compared to comparable radioisotope generators.

SUMMARY OF INVENTION In view of the above, it is an object of this invention to provide a novel plutonium radioisotope heat source having long life and high reliability.

It is a further object of this invention to provide a plutonium radioisotope heat source which is highly radiologically and biologically safe.

It is a further object of this invention to provide such a heat source which may be utilized in a thermoelectric generator.

It is a further object of this invention to provide a new method for making such a heat source.

Various other objects and advantages will appear from the following description of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims. It will be understood that various changes in the details, materials and arrangements of the parts, which are herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art.

The invention relates to a radioisotope heat source and method of making same in which plutonium dioxide shards are mixed with yttrium particles inside a sealed tantalum alloy layer or layers and heat treated at a temperature of from about 1570K to about l72OK for at least about 1 hour and then encased in an outer nickel alloy clad body.

DESCRIPTION OF DRAWING The invention is illustrated in the accompanying drawing wherein:

FIG. 1 is a crosssectional side view of a thermoelectric generator incorporating the heat source of this invention;

FIG. 2 is a cross-sectional view of the radioisotope heat source utilized in the generator of FIG. 1;

FIG. 3 is a graph showing the hardness of the tantalum alloy liner walls without the treatment prescribed by the present invention; and

FIG. 4 is a graph showing the hardness of the tantalum alloy liner across its cross section after the heat source has been subjected to certain heat treatments and operations.

DETAILED DESCRIPTION It is the purpose of this invention to provide a radioisotope thermoelectric generator having a lifetime of up to about 15 to 16 years with an electrical output of about 25 milliwatts or more in a radiologically safe configuration. Such may be provided using the radioisotope powered thermoelectric generator 10 shown in FIG. 1. Generator 10 may include a radioisotope heat source 12, which will be described more fully with respect to FIG. 2, suitably supported within a cavity in a cylindrical insulator block 14 and an outer cladding 16. Heat source 12 may be maintained under pressure against a suitable planar thermocouple arrangement or thermopile 18 by a spring biasing member 20 through an insulating plug 22 which is supported in a bore of insulator block 14 at one end of the generator 10. A transition member 24 is positioned intermediate the heat source 12 and thermopile 18 to provide good thermal contact therebetween whileelectrically insulating the thermopile from the heat source. The thermopile 18 is supported by an end wall 26, mounting block 28 and transition member 30, with the end wall 26 suitably attached to the cladding 16 to provide a rigid support for the mounting block 28 and transition member 30. Transition member 30, like transition member 24, provides good thermal conductivity to mounting block 28 while electrically insulating the thermopile from the mounting block. Electrical connection may be made to the thermocouples in the thermopile 18 by a suitable electrical connector 32 and leads or circuit board 33. The interior of generator may be suitably evacuated or pumped down and then back filled with an inert gas like Xenon to stabilize thermal conductivities through pinch-off tube 35.

In order to achieve the milliwatt electrical power requirements, the thermocouples utilized in the thermopile 18 may require an extreme aspect ratio (that is cross-sectional area to length ratio) as determined by resistivity, Seebeck coefficients, temperature differential, load voltage, and electrical power requirements. A typical thermopile l8 arrangement is shown in US. Pat. application Ser. No. 290,685 to Paul Wilcox for Thermocouple and Method of Making Same, filed Sept. 20, 1972, now U.S. Pat. No. 3,821,053. In this thermopile the thermocouples are high resistivity silicon-germanium alloys formed into wafers about .18 millimeters (mm) thick and 2.5 by 3.8 mm in size. The wafers are bonded together in a stack by suitable glass material using appropriate interconnections at opposite ends of the wafers to form the desired number of thermocouples. In the generator 10 being described, one or two stacks of about 26 wafers each may be utilized and bonded together to provide 25 milliwatts of electrical energy from each stack with a thermal differential of about 700K.

As stated above, the heat source 12 is pressed into firm contact with the thermopile 18 through transition member 24 by the spring biasing member 20. In order to maintain the proper thermal contact over the lifetime of the generator 10 between the heat source and thermopile, the spring biasing member 20 should be designed to maintain a relatively constant spring pressure even if the insulator plug 22 should be dimensionally unstable to some degree. Other biasing arrangements can be utilized including such as a tension rod or combination of tension rods which are positioned about the heat source 12 and mechanically coupled thereto and to the base plate 26 to provide the desired compressive forces which are constant over generator lifetime. The transition members 24 and 30 may be discs of boron nitride, glass, or ceramics which provide the desired thermal conductivity and electrical insulation. Boron nitride may be utilized as a block or by being vapor deposited on a suitable substrate, such as thin wafers of gold, copper or alloys of such as nickel or the like, and similar arrangements.

The electrical interconnections between the thermopile 18 and the connector 32 may be made with suitable wire leads of materials such as gold or gold-plated alloys or may be provided by use of a printed-circuit type of interconnection and conductor system using such as alumina substrates with chrome-gold or plati- The heat source 12, as shown in greater detail in FIG.

2, includes a radioisotope containing heat source material or fuel mixture 34 which may be enclosed within a sealed liner or container 36 with or without an additional strength member 38. The liner 36 and strength member 38 may in turn be enclosed and nested within a sealed outer cladding 40 to provide a complete heat surce 12 which is capable of containing the radioactive material for extended periods of time, for example up to 16 years or more, without release of radioactive material due to degradation of the container for normal operating conditions or from any accident conditions which may possibly be incurred. The liner 36, strength member 38 and cladding 40 may be formed of the same general configuration and shape with a generally hollow cylindrical body portion 42 which is closed at one end by an arcuate or hemispherical shaped end portion 44, which may be somewhat flattened if desired, and at the other end by a planar end cap suitably welded to the cylindrical body portion 42. The configuration shown minimizes the number of welds which have to be provided and permits the cylindrical portion 42 and arcuate or hemispherical portions 44 to be formed by relatively simple and low cost hydroforming or deep drawing techniques with limited machining operations. Such a configuration and the proper forming thereof may also minimize the working and mechanical stress points with may normallyoccur in machined and otherwise formed members and which may often act as a failure point under long term usage in high temperature environments with materials which may tend to oxidize, reduce or otherwise attack the container materials.

Because of the possible proximity of the heat source 12 to radiation sensitive materials, such as electronics, and to people, it is desirable to use a radioisotope which requires little, if any, radioactive shielding. An alpha-emitting isotope, such as plutonium-238, is preferred due to its low or nonexistent emission of beta and gamma radiation and because of its relatively high power density and long half-life. Plutonium-238 has a half-life of about 88.4 years and in the dioxide form may have a specific power of about 0.4 watts/gram. In order to reduce the reactivity of the plutonium-238 radioisotope with containment materials, it is also preferred that the isotope be in a cermet or ceramic form. A high power density, it has been found, may be achieved using plutonium oxides, principally, at least initially, in the form of plutonium dioxides, though carbides and nitrides may provide similar power densities under some operating conditions.

Should the integrity of the heat source 12 containment layers be compromised, it is desirable that the isotope material 34 be in a form which minimizes the production of fines (that is particles less than about 3.4 micrometers in size) which are particularly hazardous to people because of the potential for accidental inhalation. The plutonium oxide fuel in material 34 may be formed as a solid, pressed pellet or as irregular shaped particles referred to as shards having random sizes (as determined by sieving) of from about 50 to 500 micrometers. The shards may be produced with much less handling and consequently far less cost than other plutonium fuel forms. It has been found that the shards, even though they are in particle rather than solid form, produce no more fines than single pellets or pelletized fuel and are easier to form and load into the heat source 12. Plutonium oxide fuel which is 80% enriched in the plutonium-238 isotope, may typically emit about 14,000 neutrons/gram-second and gamma at a peak of about 6 rads/year. The neutron flux may be reduced to about 3,500 to 4,000 neutrons/gram-second by sintering the plutonium oxide shards in a flow of oxygen-16.

In addition of plutonium oxides in material 34, a gettering material, such as yttrium or hafnium metal parti cles which are the most stable oxide forming agents compatible with the materials used for liner 36 and strength member 38, may be mixed therewith. The plutonium oxides may be from about 96.8 to 92.9 weight percent as plutonium dioxide with from about 3.2 to 7.1 weight percent of the gettering material. Yttrium is preferred as the getter material as it is easy to handle in air and is more effective as a getter for hydrogen, carbon, and nitrogen which may be present in small quantities as organic impurities in the fuel material 34. The gettering material should preferably be in a particle or chip form, and with dimensions of about 3.0 by 3.0 by .25 mm or smaller. Larger pieces or metal foils of the gettering material may not provide sufficient reduction of the plutonium oxide fuel.

Materials for liner 36, strength member 38 and cladding 40 must take into consideration the corrosion and oxidation resistance of the materials in the particular environments to which they may be subjected, as well as mechanical properties such as tensile strength at temperatures (above 1200K) and impact resistance, fabricability and weldability. The liner 36 and strength member 38 must be resistant to corrosion and oxidation which may be caused by heat source material 3 4 and impurities enclosed therein while the cladding 40 should be resistant to corrosion or other attack from materials or environment to which the outside of heat source 12 may be subjected, such as from fresh and sea water, air, and the like. With respect to this, the container materials should be capable of functioning for the lifetime of the heat source at its operating temperatures as well as at excessive temperatures caused by fire, reentry from space and the like and still provide high impact resistance should the heat source 12 strike an object at high speed even after being utilized over its lifetime at normal operating conditions and in addition being subjected to abnormal or accident conditions.

The liner 36 may act as a sacrificial layer which is of sufficient thickness to react with whatever impurities or other materials may be enclosed within the heat source in the radioisotope fuel material 34 over the lifetime of the heat source 12 so that whatever deleterious effects which may occur from such reactions will adversely effect only the liner 36. The liner 36 is preferably made of the same material as the strength member 38 to simplify compatibility or may be incorporated into a single container. It has been found that a material which will provide the long life characteristics desired for the heat source is tantalum base solid solution alloys, and particularly the tantalum-tungsten-hafnium alloy of about 90 w/o tantalum, 8 w/o tungsten and 2 w/o hafnium. Other similar alloys may be useable in heat sources but with somewhat lower strengths, lifetimes, and the like. It has been discovered that even though such tantalum alloys tend to absorb oxygen at elevated temperatures and thus become embrittled, the difficulty is overcome when formed in accordance with this invention, and they may be utilized for extended periods of time without significant degradation from the fuel mixture. The liner 36 of this material is typically deep drawn to a thickness of about 0.5 mm in the configuration shown and is provided with a cover 36;: of similar thickness and same material which is welded thereto. The strength member 38 may be comparably formed into a similar shape in which the liner 36 may be nested but with a thickness of from about 1 to about 1.5 mm which in turn is closed by an end cap 38a of similar thickness and same material welded thereto.

The cladding material for cladding 40 may be a high temperature corrosion resistant material or alloy, such as certain nickel base alloys, having the desired formability and weldability. It has been found that a suitable alloy having these desired characteristics may include about l4.5l6.5% chromium, l5-l7% molybdenum, 3-4.5% tungsten, 47% iron, 2.5% cobalt with small amounts of Mn, Si, V, C, P and S and the balance of about 55% being nickel. The cladding can be deep drawn into the generally hollow cylindrical and an arcuate or hemispherical end closed shape into which the strength member 38 may be :nested and the open end closed and sealed by a similar cladding end cap 40a. The cladding thickness may typically be from about 0.5 to 1.5 mm thick.

The three-layer capsule or heat source arrangement described is desired, not only for safety reasons, but for fabrication and handling reasons as well. With a threelayer heat source, there are two layers between the exterior environment and the decontaminable outer surface of the liner 36. Such may be preferred even though the materials recited exhibit a high degree of resistance to contamination or attack by the other materials and the environment to which the heat source 12 may be subjected when the heat source is treated as described below. If the materials are attacked even at a reduced or low rate over the lifetime of the heat source, it may be compensated for by providing a sufficient thickness of container materials to survive all potential environments to which they may be subjected. The thicknesses noted above provide more than an adeuqate amount of containment material to provide this operation over a 16 year lifetime of the heat source.

In the assembly of heat source 12, after each of the container elements and end caps have been appropriately formed, the desired mixture of getter material chips or particles and plutonium oxide shards are mixed together in the amounts and form recited above. The fuel mixture 34 is then poured or otherwise loaded into the liner 36 and a shim or disc 48 of the same material as liner 36 used to cover the fuel mixture 34 during welding. The insertion of the shim 48 into the liner 36 body tends to wipe clean a portion of the inner surface of the liner 36 of residual fuel material aiding in weld integrity and leaves a space or void 50 at the open end of liner 36. The end cap 36a may then be welded in place over the open end of liner 36. Space 50 provides an area into which helium from radioactive decay of the plutonium fuel or other gases may be collected over the lifetime of the heat source. This volume, for

a heat source having about 12 grams of plutonium, should be about 2.7 cubic centimeters. The liner'36 and its end cap 36a may then be cleaned by appropriate means to thoroughly decontaminate its outer surface of any of the fuel material 34 or other contaminants. Thereafter, the sealed liner 36 may be placed and nested within the strength member 38 and sealed therein by end cap 38a and the cladding 40 and end cap 404; appropriately positioned thereabout. The various end caps may be gas tungsten arc welded in place in an appropriate manner to provide a gas tight seal.

It has been found that if the heat source 12, prepared as stated above, is utilized at a temperature of about 1 170K for 60 days, the liner 36 at the liner-fuel interface begins to absorb oxygen into the liner material and increases in hardness as indicated by curves 52 and 54 in FIG. 3. As the use of the heat source continues, the absorption of oxygen into the liner material progresses through the liner, progressively decreasing its ductility. This oxygen is released from the fuel material 34 and from any contaminants that may have remained therein.

It has been found, however, that this oxygen absorption and container material ductility reduction may be drastically reduced by a pretreatment which includes heating the assembled fuel material 34 with liner 36 and/or strength member 38 at a temperature of from about 1570K to about 1720K for about 1 hour. The desirable effects of this pretreatment begin to decrease at temperatures below about 1570K and above about 1720K. Pretreatment below 1570K requires excessive periods of time which may result in excessive and undesirable grain growth in the container materials. Above 1720K temperature fluctuation could lead to melting of the getter. The improved operation is illustrated in FIG. 4 in which curve 56 shows the hardness of the liner material across its cross section after the heat source has been heat treated at 1670K for 1 hour. Curve 58 illustrates the hardness of a similar liner which was heat treated at 1720K for one hour and then operated for 60 days at 1 170K. Curve 60 illustrates another liner which was heat treated at 1570K for 1 hour and then operated for 60 days at 1170K while curve 62 shows a liner treated at 1670K for 1 hour and then operated for 60 days at 1 170K. As shown, this pretreatment effectively reduces the oxygen reactivity within the capsule before oxygen has time to diffuse into the liner 36 and other container materials. In addition, curve 56 illustrates that little or no oxygen absorption results from the pretreatment itself.

A heat source having thermal power of about 6 watts from 10.4 grams of plutonium-238 may produce about 25 electrical milliwatts per channel or milliwatts total output over a 16 year service life with the heat source operating at a nominal temperature of about 700K. The heat source 12 may be about 2.3 centimeters in diameter and about 2.3 centimeters long and have a weight of about 70 grams. The plutonium-238 isotope fuel is initially PuO in the form of irregular ceramic shards having a size range of from about 50 to 500 micrometers. Using the recited liner 36 and strength member 38 materials, operating temperatures of even 900K may be utilized for similar lifetimes with some modification of other generator materials and configurations.

Heat sources as described above have been impacted against a hardened tool-steel surface at angles of 45 or more and at velocities of from 100 meters per second to 300 meters per second without release of fuel from their interiors. In addition, heat source strength members have been exposed to a 1270K fire for 1 hour without any rupturing thereofleven when the interior is pressurized with helium at end-of-life concentrations.

What is claimed is:

1. Method for producing a radioisotopic heat source comprising mixing from about 96.8 to 92.9 w/o plutonium dioxide shards of from about 50 to 500 micrometers in size with from about 3.2 to 7.1 w/o of yttrium getter material; sealingly enclosing said mixture in a first container comprising about w/o Ta, 8 w/o W, 2 w/o Hf alloy; heating said first sealed container and mixture to a temperature of from about 1570K to about 1720K for about one hour; and thereafter ne sting and sealingly enclosing said first container in a second container comprising about 55% Ni, 16% Cr, 16% Mo, 4% W, 5% Fe, 2.5% Co alloy.

2. The method of claim 1 wherein said getter material comprises chips of about 3 by 3 by .25 mm in size.

3. The method of claim 1 including, nesting said first container in an additional container of the same alloy as said first container, and subjecting both said first and additional containers to said heating, and nesting and sealingly enclosing said additional container concomitantly with said first containers.

4. The method of claim 1 including, subsequent to inserting said fuel mixture into said first container, inserting an interfitting retainer member of said first container alloy into said first container to wipe said fuel mixture from interior surfaces of the first container and retain the same therein, and provide a void above said retainer member.

5. A radioisotope heat source comprising an outer sealed container of about 55% Ni, 16% Cr, 16% Mo, 4% W, 5% Fe, 2.5% Co alloy; an inner sealed container of about 90 w/o Ta, 8 w/o Hf alloy nested inside said outer container; and a mixture of from about 96.8 to 92.9 w/o plutonium dioxide shards with a yttrium getter material at least partially filling said inner container, inner walls of said inner container embodying the enhanced characteristics resulting from subjection of said inner container and said mixture to heating to temperature of from about 1570K to about 1720K for a period of about 1 hour.

6. The heat source of claim 5 wherein said containers include a hollow cylindrical portion terminating in an arcuate shaped end and a planar end welded to said cylindrical portion.

7. The heat source of claim 5 including an additional container of about 90 w/o Ta, 8 w/o W, 2 w/o Hf alloy nested inside said outer container intermediate said inner and outer containers.

8. The heat source of claim 7 wherein said mixture fills all of the arcuate shaped end and part of said cylindrical portion of said inner container and includes a retainer member of 90 w/o Ta, 8 w/o W, 2 w/o Hf alloy interfitting into said cylindrical portion against said mixture.

9. The heat source of claim 8 for use in a thermoelectric generator having a lifetime of about 16 years wherein said mixture includes about 12 grams of plutonium, said outer container has a wall thickness of about .5 mm, said intermediate container has a wall thickness of about 1.0 mm, and said inner container has a wall thickness of about .5 mm.

10. The heat source of claim 5 wherein said getter material comprises chips of about 3 mm by 3mm by .25

mm in size. 

1. Method for producing a radioisotopic heat source comprising mixing from about 96.8 to 92.9 w/o plutonium dioxide shards of from about 50 to 500 micrometers in size with from about 3.2 to 7.1 w/o of yttrium getter material; sealingly enclosing said mixture in a first container comprising about 90 w/o Ta, 8 w/o W, 2 w/o Hf alloy; heating said first sealed container and mixture to a temperature of from about 1570K to about 1720K for about one hour; and thereafter nesting and sealingly enclosing said first container in a second container comprising about 55% Ni, 16% Cr, 16% Mo, 4% W, 5% Fe, 2.5% Co alloy.
 2. The method of claim 1 wherein said getter material comprises chips of about 3 by 3 by .25 mm in size.
 3. The method of claim 1 including, nesting said first container in an additional container of the same alloy as said first container, and subjecting both said first and additional containers to said heating, and nesting and sealingly enclosing said additional container concomitantly with said first containers.
 4. The method of claim 1 including, subsequent to inserting said fuel mixture into said first container, inserting an interfitting retainer member of said first container alloy into said first container to wipe said fuel mixture from interior surfaces of the first container and retain the same therein, and provide a void above said retainer member.
 5. A RADIOISOTOPE HEAT SOURCE COMPRISING AN OUTER SEALED CONTAINER OF ABOUT 55 NI, 26% MO, 4% W, 5% FE, 2.5% CO ALLOY AN INNER CONTAINER OF ABOUT 90 W/O TA, 8 W/O HF ALLOY NESTED INSIDE SAIDOUTER CONTAINER AND A MIXTURE OF FROM ABOUT 96.9 W/O PLUTONIUM DIOXIDE SHARDS WITH A YTTRIUM GETTING AT LEAST PARTIALLY FILLING SAID INNER CONTAINERR, INNER WALLS OF SAID INNER CONTAINER EMBODYING THE ENHANCED CHARACTERISTICS RESULTING FROM SUBJECTION OF SAID INNER CONTAINER AND SAID MIXTURE TO HEATING TO TEMPERATURE OF FROM ABOUT 1570K TO ABOUT 1720K FOR A PERIOD OF ABOUT 1 HOUR
 6. The heat source of claim 5 wherein said containers include a hollow cylindrical portion terminating in an arcuate shaped end and a planar end welded to said cylindrical portion.
 7. The heat source of claim 5 including an additional container of about 90 w/o Ta, 8 w/o W, 2 w/o Hf alloy nested inside said outer container intermediate said inner and outer containers.
 8. The heat source of claim 7 wherein said mixture fills all of the arcuate shaped end and part of said cylindrical portion of said inner container and includes a retainer member of 90 w/o Ta, 8 w/o W, 2 w/o Hf alloy interfitting into said cylindrical portion against said mixture.
 9. The heat source of claim 8 for use in a thermoelectric generator having a lifetime of about 16 years wherein said mixture includes about 12 grams of plutonium, said outer container has a wall thickness of about .5 mm, said intermediate container has a wall thickness of about 1.0 mm, and said inner container has a wall thickness of about .5 mm.
 10. The heat source of claim 5 wherein said getter material comprises chips of about 3 mm by 3mm by .25 mm in size. 